Processes for preparing vinyl chloride polymers

Abstract: Vinyl chloride polymer is prepared by suspension polymerization of monomeric vinyl chloride or a mixture of vinyl chloride and another monomer in an aqueous medium while agitating the suspension. A first partially saponified polyvinyl alcohol having a degree of saponification of 60 - 80 mol% and an average degree of polymerization of 500-1,000 is added as a suspension stabilizer at the start of polymerization, and a second partially saponified polyvinyl alcohol having a degree of saponification of 75 - 85 mol% and an average degree of polymerization of 1,500-2,700 is added while the conversion is 30% to 60%. The agitating power is controlled to 80 - 120 kg·m/s·ton from the start of polymerization to a conversion of 20 to 50%, and to 130-200 kg·m/s·ton from then to the completion of polymerization. The resulting vinyl chloride polymer is of quality as demonstrated by a high bulk density and improved free flow, monomer removal, gelation, and plasticizer absorption as well as minimal fish eye. Scale build-up on the reactor wall is minimized.

Description: This specification relates to processes for preparing vinyl chloride polymers. Vinyl chloride polymers are useful resins having excellent physical properties and find widespread application whether they are rigid or flexible. Vinyl chloride polymers are generally shaped by calendering, extrusion molding and injection molding techniques. In an advanced rigid extrusion molding technique, it is desired to increase the throughput of a molding machine. It is then desired to have a vinyl chloride polymer having a high bulk density. In preparing vinyl chloride polymers by suspension polymerization in aqueous media, many attempts were made to produce vinyl chloride polymers having a higher bulk density, for example, by feeding an addition monomer during polymerization as disclosed in Japanese Patent Application Kokai (JP-A) No. 16800/1984, using highly saponified polyvinyl alcohol as disclosed in JP-A 7600/1982, and feeding an additional suspending agent during polymerization as disclosed in JP-A 39309/1993 or USSN 922,187. Although these processes are successful in producing polymers having a high bulk density, the polymers are less porous, so that moulded parts have many "fish eye" defects and gelation is difficult. The low porosity also hinders monomer removal, which means that the polymer grains have a high concentration of residual unreacted monomer, with bad effect on the working environment associated with polymer preparation or molding and working processes. There is a possibility that the unreacted monomer be left in molded parts, and the presence of residual monomer is detrimental in a particular application associated with food. On the other hand, improvements were made in polyvinyl alcohol as the suspending agent. There were developed oil-soluble polyvinyl alcohols having a low degree of polymerization and a low degree of saponification and modified polyvinyl alcohols having various modifying groups introduced therein. Among them, the oil-soluble polyvinyl alcohols are effective for improving monomer removal and fish eye control, but result in a lowering of bulk density and deteriorated free flow due to electrostatic charging. The modified polyvinyl alcohols fail to keep suspension systems stable with conventional techniques, leaving the problems of scale build-up and substantial particle size variation. JP-A 311708/1992 or USSN 08/060,596 proposes a dispersion polymerization method by which a vinyl chloride polymer having comparatively high bulk density, low monomer removal, good plasticizer absorption and minimized fish eyes was obtained with minimizing scale build-up in a reactor. However, the bulk density of the resulting polymer is not so fully improved from the standpoint of an extruder output in extrusion molding. Therefore, there is still scope for development in obtaining vinyl chloride polymers having a high bulk density. FR-A-2207149 (equivalent to US-A-3929753) discloses the use of two different saponified polyvinyl alcohol suspending agents at different stages of the polymerisation. The first agent is 70 to 80% saponified with polymerisation degree 500 to 900. The second agent is 75 to 95% saponified with polymerisation degree from 1000 to 3000. The second agent is added at some stage between 3 and 40% conversion, for example 30%. The general aim of this procedure is to increase bulk density and improve gelling and processing properties. The first suspending agent used in the examples is Gosenol KP-08 whose light absorbance at 280 nm in 1% aqueous solution is 2.9 to 3.2 (although this fact is not mentioned in the document). A general aim herein is to provide new and useful processes for making vinyl chloride polymers with high bulk density. Preferred aims include, independently: the reduction of fish eye defects; achieving free flow; achieving low residual monomer content; achieving good gelation; achieving good plasticiser absorption; achieving low scale build-up in the reactor. We have investigated polymerisation conditions in suspension polymerisation processes carried out in aqueous medium with agitation, on a monomer charge comprising vinyl chloride. We found that advantageous results, including high bulk density and also the various preferred aims explained above, could be achieved in processes in which we (i) used partially saponified polyvinyl alcohol as a suspension agent, and increased during the process the degree of polymerisation, and preferably also the degree of saponification, of the suspension agent, and (ii) increased during the process the vigour of agitation applied to the suspension. Feature (i) was satisfied using initially a partially saponified polyvinyl alcohol having a degree of saponification of 60 to 80mol% and an average degree of polymerisation of from 500 to 1000, and progressing to partially saponified polyvinyl alcohol having a degree of saponification of 75 to 85mol%, and preferably a higher degree of saponification than the agent used initially, and an average degree of polymerisation of from 1500 to 2700. Good results were obtained when the initially-used polyvinyl alcohol had a viscosity of up to 15 centipoise measured in 4% aqueous solution at 20°C, and a light absorbance of at least 4 measured in 1% aqueous solution at a wavelength of 280 nm. The latter type of polyvinyl alcohol suspension agent is added to the suspension at an intermediate stage of polymerisation, between 30% and 60% conversion. Most simply, a suspension agent as first defined is included at the beginning for a first phase of polymerisation, and a suspension agent as second defined is then added at the intermediate stage, for a second phase of polymerisation. To achieve feature (ii), agitation power may be maintained at a first, lower level until an intermediate stage of polymerisation, 20 to 50% conversion, and then at a higher level thereafter e.g. to the completion of polymerisation. Agitation power for the first level is 80 to 120kg.m/s.tonne and at the higher level is from 130 to 200kg.m/s.tonne. To explain briefly, the suspension stabiliser used at the initial stage of polymerisation is a partially saponified polyvinyl alcohol having relatively intense emulsifying/dispersion action. If a greater agitation power is given at the initial stage of polymerization in a system using such a suspension stabilizer having enhanced emulsifying/dispersing action, the monomer liquid is dispersed into too fine droplets so that the suspension system becomes unstable, giving rise to the problem that the polymer forms coarse grains and much scale deposits on the reactor wall. Therefore, the agitating power should be controlled relatively low at the initial stage of polymerization. In a later stage, a highly saponified polyvinyl alcohol is added for increasing the bulk density of polymer particles, and the agitating power is increased for thereby depriving the polymer grains of microparticulates adhering to their surface. Then a vinyl chloride polymer of quality can be produced without depositing scale on the reactor wall. Accordingly, we specifically propose a process for preparing a vinyl chloride polymer by suspension polymerization of a monomeric charge containing vinyl chloride in an aqueous medium while agitating the polymerizing suspension. The monomeric charge consists of monomeric vinyl chloride or is a mixture of monomeric vinyl chloride and at least one vinyl monomer copolymerizable therewith. (A) A first partially saponified polyvinyl alcohol is added to the suspension as a suspension stabilizer at the start of polymerization. The first saponified polyvinyl alcohol has a degree of saponification of 60 to 80 mol%, an average degree of polymerization of 500 to 1,000, a viscosity of up to 15 centipoise as measured in 4% aqueous solution at 20°C, and a light absorbance of at least 4 as measured in 1% aqueous solution at a wavelength of 280 nm. (B) A second partially saponified polyvinyl alcohol is added to the suspension in an amount of 0.001 to 0.5% by weight based on the monomeric charge while the conversion is 30% to 60%. The second saponified polyvinyl alcohol having a degree of saponification of 75 to 85 mol% and an average degree of polymerization of 1,500 to 2,700. (C) In a duration from before the start of polymerization to a conversion of 20 to 50%, the agitating power is 80 to 120 kg·m/s·ton. (D) In a duration from thereafter to the completion of polymerization, the agitating power is 130 to 200 kg·m/s·ton. EXPLANATIONS; OPTIONAL AND PREFERRED FEATURES As explained above, we are concerned with a process, typically a batch process, for preparing a vinyl chloride polymer by suspension polymerization of a monomeric charge containing vinyl chloride in an aqueous medium while mechanically agitating the suspension. The charge may consist substantially entirely of monomeric vinyl chloride. Alternatively the monomeric charge is a mixture of monomeric vinyl chloride and at least one vinyl monomer copolymerizable therewith. The mixture preferably contains more than 50% by weight, preferably more than 80% by weight of monomeric vinyl chloride. Examples of the comonomer used herein include vinyl esters such as vinyl acetate and vinyl propionate; acrylates and methacrylates such as methyl (meth)acrylate and ethyl (meth)acrylate; olefins such as ethylene and propylene; vinyl ethers such as lauryl vinyl ether and isobutyl vinyl ether; and other monomers copolymerizable with vinyl chloride such as maleic anhydride, acrylonitrile, styrene, and vinylidene chloride. One or more of these comonomers may be used in combination with vinyl chloride. To the monomeric charge is added a polymerization initiator, which may be selected from conventional ones used for the suspension polymerization of prior art vinyl chloride systems. Exemplary polymerization initiators which can be used herein include percarbonates such as diisopropylperoxydicarbonate (IPP), di-2-ethylhexylperoxydicarbonate, and diethoxyethylperoxydicarbonate; peresters such as t-butylperoxyneodecanate, t-butylperoxypivalate, t-hexylperoxypivalate, α-cumyl-peroxyneodecanate, and 2,4,4-trimethylpentyl-2-peroxy-2-neodecanate; peroxides such as acetylcyclohexylsulfonylperoxide, 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate, 3,5,5-trimethylhexanoylperoxide, and lauroyl peroxide (LPO); azo compounds such as azobis-2,4-dimethylvaleronitrile and azobis(4-methoxy-2,4-dimethylvaleronitrile) alone or in admixture of two or more. Preferred polymerization initiators are percarbonates and peresters. Preferably the polymerization initiator is added to the reaction system in an amount of about 0.03 to about 0.2 parts by weight per 100 parts by weight of the monomeric charge. After a reactor is loaded with the monomeric charge, the initiator may be added to the reaction system by diluting it with a suitable solvent or forming an aqueous emulsion and pumping the dilution or emulsion to the reaction system under pressure. Polymerisation conditions other than those peculiar to the novel techniques proposed herein, such as charging method, charging proportion and polymerization temperature, may be in accord with conventional ones. Preferably the weight ratio of water to monomer is from 0.9:1 to 1.5:1, more preferably from 1.0:1 to 1.25:1 at the initial charging. Additional water may be added during polymerization if desired. To reduce the heating time and stabilize the suspension system, use of deionized water at a temperature of 40 to 50°C is preferred. If the weight ratio of water to monomer is less than 0.9, the suspension stability of the reaction system is lowered, resulting in a broader particle distribution of the resulting polymer, the formation of coarse particles, and much scale build-up in the reactor. On the other hand, if the weight ratio of water to monomer is more than 1.5, a bulk density of the resulting polymer is not so fully improved. The suspension stabilizer used at the start of polymerization and forward is a first partially saponified polyvinyl alcohol having a degree of saponification of 60 to 80 mol%, an average degree of polymerization of 500 to 1,000, a viscosity of up to 15 centipoises as measured in 4% aqueous solution at 20°C, and an absorbance of at least 4 as measured in 1% aqueous solution at a wavelength of 280 nm. More preferably, the first partially saponified polyvinyl alcohol has a degree of saponification of 65 to 75 mol%, an average degree of polymerization of 700 to 1,000, a viscosity of 4 to 8 centipoises as measured in 4% aqueous solution at 20°C and an absorbance of 4.5 to 7.5 as measured in 1% aqueous solution at a wavelength of 280 nm. We have found that selection within these particular parameters gives particularly good results in achieving high bulk density while avoiding fish eyes. Preferably the first partially saponified polyvinyl alcohol is added in an amount of 0.02 to 0.08% by weight of the monomeric charge. It may be introduced into the reactor by dispersing or dissolving in a suitable solvent such as water. Typically after the reactor is charged with a vinyl monomer, suspension stabilizer and other ingredients, agitation is started to uniformly disperse the contents. Desirably initial agitating power is set in the range of 80 to 120 kg.m/s.t, preferably 90 to 110 kg.m/s.t. Such initial power may be maintained until a conversion of 20 to 50%, preferably 30 to 40% is reached. Outside these recommended ranges there is a tendency for the resulting polymer to have a lower bulk density, for more scale to build up on the reactor wall, and for coarser grains to form. Thereafter, the agitator is accelerated to increase the agitating power, desirably to 130 to 200 kg.m/s.t, preferably 130 to 180 kg.m/s.t. Such power may be maintained until the completion of polymerization. If the later agitating power is lower than 130 kg.m/s.t, the resulting polymer tends to have lower bulk density and plasticizer absorption and contains a larger amount of unreacted monomers. Further, heat removal near the end of polymerization becomes difficult and when a reflux condenser is connected to the kettle, there arises an undesirable phenomenon that the polymer is carried over into the condenser. If the later agitating power exceeds 200 kg·m/s·ton, polymer grains tend to be smaller and thus less free flowing. Typically, while the conversion is from 30 to 60%, preferably from 40 to 50%, a second partially saponified polyvinyl alcohol is added to the suspension in an amount of about 0.001 to about 0.5% by weight, preferably 0.001 to 0.2% by weight, more preferably 0.001 to 0.1% by weight based on the monomeric charge, to increase bulk density. The second saponified polyvinyl alcohol typically has saponification of 75 to 85 mol% and an average degree of polymerization of 1,500 to 2,700, and preferably has a degree of saponification of 80 to 85 mol% and an average degree of polymerization of 2,000 to 2,700. If the second polyvinyl alcohol is prematurely added before a conversion of 30% is reached, polymer grains are reduced in size and bulk density and thus less free flowing. If it is added after a conversion of 60% is over, bulk density is no longer increased. A partially saponified polyvinyl alcohol having a degree of saponification of less than 75 mol% and an average degree of polymerization of less than 1,500 is not effective for increasing bulk density whereas a partially saponified polyvinyl alcohol having a degree of saponification of more than 85 mol% and an average degree of polymerization of more than 2,700 is less effective for increasing bulk density and introduces many fish eyes. Less than 0.001% by weight of the second polyvinyl alcohol is usually not effective for its purpose while more than 0.5% by weight of the second polyvinyl alcohol gives no further benefits and is thus economically undesired. The agitating power is varied in dependence on the degree of conversion in the present technique, so the reactor and agitator structures are desirably chosen to facilitate this. The reactor is preferably a cylindrical kettle having a height to diameter ratio of from 1.5:1 to 2.5:1. The preferred agitator includes multiple stages of paddles, faudlers, propellers, and turbine blades, all combined with buffles. It should be noted that ingredients commonly used in vinyl chloride systems for polymerization, for example, polymerization control agents, chain transfer agents, pH adjusting agents, gelation enhancers, antistatic agents, crosslinking agents, stabilizers, fillers, antioxidants, buffer agents, and anti-scaling agents may be added at an appropriate stage. EXAMPLE Examples of the present invention are given below by way of illustration. Examples 1-2 & Comparative Examples 1-5 A stainless steel polymerization kettle with an interior volume of 2.1 m 3 equipped with an agitator and a jacket was charged with 845 kg of deionized water at 45°C and the suspension stabilizer shown in Table 1 in the amount shown in Table 1 and then evacuated until a vacuum of 50 mmHg was reached. Then the kettle was charged with 760 kg of monomeric vinyl chloride. While agitating the contents under the conditions shown in Table 1, hot water was passed through the jacket to start heating of the contents. At the same time, 420 g of di-2-ethylhexylperoxydicarbonate was admitted into the kettle under pressure to start polymerization. Then polymerization reaction was continued while keeping the suspension at a temperature of 57°C. Initially and during polymerization, suspending agents as shown in Table 1 were added and the agitation conditions were adjusted as shown in Table 1. At the time when the pressure within the kettle reached 6.0 kg/cm 2 G, the unreacted monomer was recovered and the polymer in slurry form was taken out of the kettle and dried. There was obtained a vinyl chloride polymer. Comparative Example 6 A stainless steel polymerization kettle with an interior volume of 2.1 m 3 equipped with an agitator and a jacket was charged with 845 kg of deionized water at 45°C and the suspension stabilizer shown in Table 1 in the amount shown in Table 1 and then evacuated until a vacuum of 50 mmHg was reached. Then the kettle was charged with 760 kg of monomeric vinyl chloride. While agitating the contents under an agitating power of 120 kg·m/s·ton, hot water was passed through the jacket to start heating of the contents. At the same time, 420 g of di-2-ethylhexylperoxydicarbonate was admitted into the kettle under pressure to start polymerization. Then polymerization reaction was continued while keeping the suspension at a temperature of 57°C. On the way of polymerization, the agitating power was changed to 110 kg·m/s·ton at the conversion of 10%, 100 kg·m/s·ton at the conversion of 20% and 140 kg·m/s·ton at the conversion of 60% as shown in Table 1. At the time when the pressure within the kettle reached 6.0 kg/cm 2 G, the unreacted monomer was recovered and the polymer in slurry form was taken out of the kettle and dried. There was obtained a vinyl chloride polymer. The vinyl chloride polymer was measured for bulk density, particle size distribution, DOP absorption, fish eye, residual monomer, and gelation by the following methods. The kettle interior was visually observed for scale build-up. The results are also shown in Table 1. Bulk density It was measured according to JIS K-6721. Particle size distribution It was measured according to JIS Z-8801. The particles were sifted through #60, #100 and #200 screens and the passages were determined in % by weight. DOP absorption An aluminum alloy container having an inner diameter of 25 mm and a depth of 85 mm was packed with fiber glass on the bottom. The vinyl chloride polymer, 10 grams, was weighed and admitted into the container. Dioctyl phthalate (DOP), 15 cc, was added to the resin which was allowed to stand for 30 minutes, allowing the DOP to fully penetrate into the resin. Thereafter, excess DOP was centrifuged under an acceleration of 1500G. The amount of DOP absorbed in the resin was determined and expressed in parts by weight per 100 parts by weight of the resin. Fish eye A mixture of 100 parts of the vinyl chloride polymer, 50 parts of DOP, 0.1 parts of barium stearate, 0.1 parts of cadmium stearate, 0.8 parts of cetyl alcohol, 2.0 parts of a tin stabilizer, 0.5 parts of titanium dioxide, and 0.1 parts of carbon black, all in parts by weight, was milled on a 6'' (152mm) roll mill at 140°C for 5 minutes to form a sheet of 0.3 mm thick. The number of white transparent particles in a 100-cm 2 area of the sheet was counted. Residual monomer After a certain amount of the vinyl chloride polymer was dissolved in tetrahydrofuran, the monomer content in the polymer was quantitatively determined by gas chromatography. The amount of residual monomer is expressed in ppm based on the weight of dry polymer. Gelation In a Henschel mixer, 100 parts of the vinyl chloride polymer was blended with 0.5 parts of tribasic lead sulfate, 2.5 parts of lead stearate, and 0.7 parts of barium stearate, all in parts by weight. The mixture, 67 grams, was admitted into a Brabender Plasti-Corder® and milled at 210°C and 40 rpm. The gelation time was the time taken until a maximum torque was reached. Scale build-up in the polymerization kettle After the slurry was taken out of the kettle, the scale build-up on the interior wall was visually inspected and rated according to the following criterion. o ○: metal surface maintained mirror finish luster without scale deposit ○: cloudy mirror finish metal surface × : filmy scale over the entire metal surface Suspension stabilizer A Partially saponified polyvinyl alcohol having a degree of saponification of 72.4 mol%, an average degree of polymerization of 770, a viscosity of 5.7 centipoise as measured in 4% aqueous solution at 20°C, and an absorbance of 6.5 as measured in 1% aqueous solution at wavelength 280 nm Suspension stabilizer B Partially saponified polyvinyl alcohol having a degree of saponification of 80.2 mol% and an average degree of polymerization of 2,600 Suspension stabilizer C Hydroxypropylmethyl cellulose having a methoxy substitution of 29.2% by weight, a hydroxypropoxy substitution of 8.9% by weight, and a viscosity of 49.5 centipoise as measured in 2% aqueous solution at 20°C Suspension stabilizer D Partially saponified polyvinyl alcohol having a degree of saponification of 72.4 mol%, an average degree of polymerization of 770, a viscosity of 5.7 centipoise as measured in 4% aqueous solution at 20°C and an absorbance of 3.2 as measured in 1% aqueous solution at wavelength 280 nm. As seen from Table 1, the present processing techniques can produce vinyl chloride polymers having a high bulk density, a low microparticulate content, few fish eyes, and minimized residual monomer while minimizing the scale deposition to the kettle wall. In contrast, if no suspension stabilizer was added during polymerization, the resulting polymer had a low bulk density although the agitating conditions were varied as prescribed herein (Comparative Example 2). If the agitating conditions were kept unchanged from the start to the end, the resulting polymer had many fish eyes and contained more residual monomer although the different stabilizer was added during polymerization as prescribed herein (Comparative Example 3). Outside the polymerizing conditions prescribed herein, much scale deposited on the kettle wall. Next, the vinyl chloride polymers obtained in Example 1 and Comparative Example 6 were tested for extruder output in the following manner. Formulation parts by weight Vinyl chloride polymer 100 Lead stabilizer 2.5 Barium stearate 0.5 Stearic acid 0.3 Polyethylene wax 0.3 The above components were charged in a 10 liter-Henschel mixer in the above amounts to mix and agitate them. When the temperature of the mixture reached 120°C, cooling water was flowed in a jacket attached to the Henschel mixer to cool the mixture in the mixer. There was obtained a rigid polyvinyl chloride compound for extrusion molding. By using a 20 mm diameter extruder, a rod having a size of 5 mm × 10 mm was extruded from the compound in the following conditions to evaluate the extruder output of the compound. The results were shown below. Extrusion conditions Screw: CR = 3.0, L/D = 20, Rotary speed = 30 rpm Cylinder temperature C 1 = 160°C C 2 = 180°C C 3 = 170°C Die temperature D = 180°C Extruder output Vinyl chloride polymer of Example 1: 43 g/min. Vinyl chloride polymer of Comparative Example 6: 39 g/min. There has been described a process for preparing a vinyl chloride polymer by adding appropriate suspension stabilizers at appropriate stages and controlling the agitation power in early and later durations. The resulting vinyl chloride polymer is of quality in that it has a high bulk density, is improved in free flow, monomer removal, gelation, and plasticizer absorption, and presents few fish eyes. The polymer is obtained without substantial scale build-up on the reactor wall. Japanese Patent Application No. 5-131274 is incorporated herein by reference. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: A process for preparing a vinyl chloride polymer by suspension polymerization of a monomeric charge containing vinyl chloride in an aqueous medium while agitating the suspension, wherein (A) a first partially saponified polyvinyl alcohol is added to the suspension as a suspension stabilizer at the start of polymerization, said first saponified polyvinyl alcohol having a degree of saponification of 60 to 80 mol%, an average degree of polymerization of 500 to 1,000, a viscosity of up to 15 centipoise as measured in 4% aqueous solution at 20°C, and a light absorbance of at least 4 as measured in 1% aqueous solution at a wavelength of 280 nm, (B) a second partially saponified polyvinyl alcohol is added to the suspension in an amount of 0.001 to 0.5% by weight based on the monomeric charge while the conversion is 30% to 60%, said second saponified polyvinyl alcohol having a degree of saponification of 75 to 85 mol% and an average degree of polymerization of 1,500 to 2,700, (C) from before the start of polymerization to a conversion of 20 to 50%, the agitating power is 80 to 120 kg·m/s·ton, and (D) from thereafter to the completion of polymerization, the agitating power is 130 to 200 kg·m/s·ton. A process of claim 1 wherein said monomeric charge consists of a vinyl chloride monomer. A process of claim 1 wherein said monomeric charge is a mixture of a vinyl chloride monomer and at least one other vinyl monomer. A process of any preceding claim wherein said first partially saponified polyvinyl alcohol is added in an amount of 0.02 to 0.08% by weight based on the monomeric charge. A process according to any one of the preceding claims in which the second polyvinyl alcohol is more highly saponified than the first.

Inventors: AMANO TADASHI, HIYAMA TADAYOSHI, AMANO, TADASHI, HIYAMA, TADAYOSHI, HIYAMA, TADAYOSHI, 3-208, SHIN-ETSU CHEMICAL
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C08F14
IPC codes: C08F14, C08F2